Physicochemical characterization and biological properties of a new toxic protein isolated from soybeans (Glycine max) is reported. The purification procedure consisted basically of ammonium sulfate fractionation, ion exchange, and affinity chromatographies, the latter being used for the removal of the seed's lectin and of its trypsin inhibitor. The highly purified protein, designated soyatoxin, is a single chain acidic protein (pI 4.4-4.6) of 21 kDa, dependent on reduced thiol groups to maintain its solubility and biological activities. The toxin is a metalloprotein containing iron, calcium, zinc, and magnesium. Soyatoxin is highly toxic to mice (LD50 7-8 mg/kg mouse body wt upon intraperitoneal injection). It produces dyspnoea, tonic-clonic convulsions, and flaccid paralysis prior to death of intraperitoneally injected mice. Furthermore, soyatoxin is immunologically related to another toxic protein (canatoxin), isolated from Canavalia ensiformis seeds, which is distinct from soyatoxin in containing 18 x 10 kDa noncovalently bound subunits. Some biological properties including acute intraperitoneal toxicity, canatoxin-like immunoreactivity, hemagglutination, trypsin inhibitory activity, induction of platelet release reaction, and aggregation displayed by soyatoxin were studied and used to differentiate soyatoxin from soybean lectin and trypsin inhibitors.